1. Field of the Invention
This invention relates to systems for sensing liquid levels and, more particularly, to systems utilizing ultrasonic transducers for determining liquid levels.
2. State of the Art
U.S. Pat. No. 3,834,233 to Willis et al. discloses a system for determining liquid level by echo ranging. The system includes a first ultrasonic transducer mounted at the top of a tank to direct acoustic wave energy down into the tank and detect an echo from the surface of the liquid contents of the tank. The distance from the first transducer to the surface is determined from a time measurement. Willis et al., in order to compensate for inaccuracies due to changes in the velocity of sound over the path the wave travels, position a second ultrasonic transducer at a fixed distance from the first transducer to detect the transmitted wave. Signals from the two detectors are processed to cancel the effects of any variation in the speed of sound.
Known transducers typically include a piezoelectric crystal sandwiched between a matching medium for improving energy transfer from the crystal to a gaseous environment and a backing for dampening ringing of the crystal (continued vibration of the crystal after excitation). The materials composing the medium and backing typically limit the temperature range at which the medium efficiently transfers acoustic energy and the backing efficiently dampens ringing.
U.S. Pat. No. 2,430,013 to Hansel discloses a matching medium positioned between a crystal and a water environment. Hansel teaches a medium thickness of an odd multiple of quarter wavelengths and the adusting of the acoustic impedance of a medium material by adding other finely divided materials including glass.
U.S. Pat. No. 3,995,179 to Flournoy discloses a backing composed of an epoxy resin having a plurality of pointed steel rods molded therein.